1. Field of the Invention
The present invention relates to a re-chargeable, nonaqueous electrolyte secondary cell, which is used as a power source for various electronic appliances, and, in particular, it relates to such a non-aqueous electrolyte secondary cell of which the cathode has been improved.
2. Prior Art
With rapid progress in various electronic appliances, the recent studies in the art are toward re-chargeable secondary cells that can be used as power sources conveniently and economically for a long period cf time. As typical secondary cells, known are lead accumulators, alkali accumulators, lithium secondary cells, etc. Of those, lithium secondary cells are especially advantageous in that their output is high and their energy density is also high.
Such a lithium secondary battery comprises a cathode of an active material capable of reversibly and electrochemically reacts with lithium ions, metal lithium or an anode containing lithium, and a non-aqueous electrolyte. The discharge reaction occurring in the lithium secondary cell is generally such that lithium ions are dissolved out from the anode into the non-aqueous electrolyte while lithium ions are intercalated in the layers of the active material of the cathode. On the contrary, where the cell is charged, reaction opposite to the discharge reaction occurs in the cell, or that is, lithium ions are de-intercalated in the cathode. Therefore, in the lithium secondary cell of that type, the charge-discharge reaction is repeated on the basis of the movement of the lithium ions which are supplied by the anode and which are absorbed and desorbed by the active material of the cathode.
As the active material for the anode of a lithium secondary cell, for example, generally used are metal lithium, lithium alloys (e.g., Li--Al alloys), lithium-doped, electroconductive polymers (e.g., polyacetylene, polypyrrole), and interlayer compounds having lithium ions as intercalated into their crystal structures. On the other hand, as the active material for the cathode of the cell, for example, generally used are metal oxides, metal sulfides and polymers. Concretely known are TiS.sub.2, MoS.sub.2, NbSe.sub.2 and V.sub.2 O.sub.5. Recently, a non-aqueous electrolyte secondary cell has been put into practical use, which comprises a cathode active material, Li.sub.x Co.sub.y O.sub.2 (in which x varies depending on charging and discharging of the cell, but, in general, x and y each are about 1 (one) just after the production of the compound), having a high discharge potential and a high energy density.
However, since cobalt which is a raw material for the composite oxide is rare on the earth, and, in addition, since ore deposits containing cobalt and capable of being used commercially locally exist only a few limited lands, cobalt is expensive and its price variation is great, and, in addition, there is a probability that cobalt will be in short supply in future. For these reasons, LiNiO.sub.2 or LiMn.sub.2 O.sub.4, which is prepared from raw materials that are more inexpensive than cobalt while existing richly on the earth, and of which the properties are comparable to those of lithium-cobalt composite oxides, is being used as the cathode active material in lithium secondary cells, thereby much popularizing the cells in a variety of fields. In particular, manganese is more inexpensive than not only cobalt but also nickel, and exists more richly than the latter on the earth. Manganese dioxide is much distributed in the market as the material for manganese dry cells, alkali manganese dry cells and lithium primary cells, and there is little probability that it will be in short supply in future. For these reasons, many studies of lithium-manganese composite oxides for the cathode active material in non-aqueous electrolyte secondary cells, which use manganese as one material, are being made in these days. Of those, spinel-structured lithium-manganese composite oxides were reported to have a potential of 3 V or higher relative to lithium and have a theoretical charge-discharge capacity of 148 mAh/g, after having been electrochemically oxidized.
However, lithium ion-containing, non-aqueous electrolyte secondary cells comprising a manganese oxide or a lithium-manganese composite oxide as the cathode active material are defective in that their properties are worsened in charge-discharge cycles. In particular, when the cells are used in high-temperature environment over room temperature, their properties are much worsened.
Especially for large-sized, non-aqueous electrolyte secondary cells for electric cars or for road leveling, the worsening of their properties is serious. This is because, for such large-sized cells, the internal heat to be generated by them in their use could not be negligible. Therefore, even if the ambient temperature around the cells being used is about room temperature, there increases a probability that the internal temperature of the cells will be relatively high. This problem should not be neglected even for relatively small-sized cells to be used in small-sized portable appliances, considering the fact that the appliances will be used in high-temperature environment such as in the rooms of cars in full summer.